Now this is a small crab:
.. many specimens imported are less than 1cm across the carapace and
therefore the anemones are much smaller still. Each one may be only a few millimetres
in diameter with very small tentacles. Not much use against a marauding predator? Well,
it depends upon what kind of predator you are talking about. Unlike the large
hermits crabs that give a lift to anemones on their shells the boxer crab can
squeeze into small crevices and is very unlikely to venture into the open for
long. Thus potential predators are likely to also be small rock dwellers such
as gobies and other small fish. The pincers of the boxer crab are so small and
well adapted to their role in holding the anemones that they are ineffectual
in defence but with the extra punch afforded by the stings of the anemones they
act as a reasonable deterrent. Interestingly boxer crabs are known to use at
least three species of anemone in this way including Triactis producta and Bundeopsis
sp, both of which are alleged to have very powerful stings for their size.
These anemones can be found in a free-living state and are therefore not dependent
on the crab, whereas the reverse is certainly not true. The boxer crab advertises
the fact that it is in possession of such useful weaponry by waving its pincers
in the general direction of any potential predator in the manner of a shadowboxing
pugilist, hence one of the common names for this species. However, despite the
fact that the anemones will be used if needed the crab could much rather avoid
any potential conflict and a quick bluffing combination of two to the imaginary
body and one to the head is deemed sufficient and the crab makes a hasty retreat.
Having a really specialised pair of pincers can be great, the fact that there
number of different species of Lybia that carry anemones is a testament
to the fact that they must be very useful and the compromise that the crab has
made by reducing the size of its pincers into no more than a couple of grippers
must have some drawbacks? Crab pincers, correctly termed chelipeds, are
useful in defence, as anyone that has received a nip will testify. The boxer
crab would appear to have defence covered.
But what about food acquisition?
strong and powerful pair of pincers is very useful at gripping but also tearing
or ripping. Many scavenging xanthid crabs closely related to Lybia use
their pincers to pull apart food into manageable chunks. Lybia’s pincers
are not strong enough to do this and should they try it would mean putting
down their anemones and rendering themselves extremely vulnerable to predation.
a result the boxer crab has adapted to use its second pair of legs, the first
pair of walking legs in this role and they are very effective at ripping off
small pieces of food from larger chunks and manipulating them delicately towards
Observations have also shown that the anemones themselves can have
a role in food capture for the crab. In this instance the crab uses
the cnidarians as “mops” in which they are wiped along
the substrate. The particulate material collected in the sticky tentacles
is then removed by the mouthparts of the crab. Either feeding method
will lead to the nourishment of both the crab and its anemones.
In an aquarium situation a variety of foodstuffs will
be accepted including chopped mussel, cockle, prawn and shrimp.
However, boxer crabs are elusive creatures and any aquarist prepared
this fascinating animal must be prepared for very sporadic sightings
unless it is the only individuals housed in a small aquarium. Many
aquarists report sightings as infrequent as once a year or less! This
to be the case in a large reef aquarium with plenty of busy fish
but hopefully more often in smaller systems. Of course, it all depends
where the crab takes up residence and its subsequent growth and
When choosing an individual make sure that it has hold of one anemone
in each pincer. Although it can seemingly replace one if it becomes
damaged or lost (the exact mechanism for this is unclear but may involve
the crab taking a cutting from a healthy individual) the first moments
after introduction will be when this crab is at its most vulnerable
and it may need both its stingers to repel any over-inquisitive species.
Needless to say this is not a crab for an aquarium containing any fish
species likely to bother crustaceans and is best suited to coral rich
aquaria with peaceful fish in residence.
Occasionally you will see boxer crabs for sale with a bright red-orange
mass on their underside. This is an egg mass carried by the female
crab (Boxer crabs have separate sexes –distinguished by the
width of their tails. Males have very thin tails whereas those of
females are much larger as they assist in holding the egg mass in
position) and although no one has succeeded in successfully raising
the planktonic young of this species through the various life cycle
stages this may be a suitable candidate for the enthusiastic hobbyists
with plenty of spare time an the ability to apply themselves to a
As we end our look at this aquarium wonder we hope
that we have tempted some of our readers to have a go at keeping this
wonderful animal in their aquarium. If not then consider this: there
are many sights in nature that we will never be lucky enough to witness
but the sight of a boxer crab moulting is one that is within the grasp
of every marine aquarist. Watch in awe as the crab carefully places
each anemone in turn in a safe place and then as quickly as possible
liberates itself of its old exoskeleton. Then it quickly grasps its
anemones in its almost vestigial pincers before retreating to a safer
place to allow the new body to harden. Wonders like this are a privilege
to behold and something that can happen in your own living room.
Ying Zhang for original article idea.
Checklist of Sea Anemones (Cnidaria: Anthozoa) From Egypt.
Presented by Dr. Fayez F. A. Shoukr, Professor of Invertebrates, Zoology
Department, Faculty of Science, Tanta University, Tanta 31527, Egypt
Clin Toxicol. 1970 Dec;3(4):637-43.
Report of stingings by the sea anemone Triactis producta Klunzinger
from Red Sea.
Levy S, Masry D, Halstead BW.